Drug disposition is a highly regulated process in mammals. For example, the expression of genes encoding drug-transporting and drug-metabolizing proteins is induced by their substrates in liver and intestine. Recent research has shed new light on the molecular mechanism of this phenomenon. The pregnane X receptor (PXR) is a ligand-activated transcription factor that is expressed in liver and intestine. PXR coordinately regulates the drug-inducible expression of drug-transporting and drug-metabolizing proteins in liver and intestine. PXR binds to xenobiotic-response elements (XREs) in the promoters of its target genes as a heterodimer with the 9-cis-retinoic acid receptor (RXR). One extremely important PXR-target gene is the cytochrome P450 3A4 (CYP3A4) gene. CYP3A4 is primarily expressed in liver and intestine where it catalyzes the oxidative metabolism of nearly 60% of all clinically prescribed drugs. The hepatic expression of CYP3A4 and other PXR-target genes is rapidly suppressed in response to inflammatory cytokines, though the molecular basis of this repression is currently unknown. Repression of PXR-target gene expression occurs in people that undergo invasive surgery in response to inflammation, thereby producing altered pharmacokinetic properties of drugs in these patients. PXR is activated by a plethora of structurally diverse molecules including drugs, steroids, bile acids, and xenobiotics. Therefore, repression of PXR-target gene expression can produce potentially life-threatening drug-drug interactions. Thus, it is important to understand the molecular basis of the repression of PXR-target genes in order to predict and prevent the occurrence of drug interactions in post-operative patients. An alteration in the phosphorylation status of transcriptional regulatory proteins is a likely mechanism that mediates the repression of PXR-target gene expression following inflammation. The goals of these investigations are to (1) identify the sites of PXR phosphorylation following activation of specific cell signaling pathways, and (2) determine the effect of increased PXR phosphorylation on it's ability to (a) transactivate, (b) bind DNA, (c) translocate from the cytoplasm to the nucleus, and (d) interact with protein co-factors. Successful completion of these studies will allow us to determine the molecular basis of the repression of key drug metabolizing genes during the inflammatory response and provide increased opportunities to understand the xenobiotic response. These studies will provide additional insight into the regulation of drug disposition and drug-drug interaction.